Glyoxal Yield from Isoprene Oxidation and Relation to Formaldehyde

Home , Glyoxal

Atmos. Chem. Phys., 17, 8725–8738, 2017 https://doi.org/10.5194/acp-17-8725-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License.

Glyoxal yield from isoprene oxidation and relation to formaldehyde: chemical mechanism, constraints from SENEX aircraft observations, and interpretation of OMI satellite data

Christopher Chan Miller1, Daniel J. Jacob1,2, Eloise A. Marais1, Karen Yu2, Katherine R. Travis2, Patrick S. Kim1, Jenny A. Fisher3, Lei Zhu2, Glenn M. Wolfe4,5, Thomas F. Hanisco4, Frank N. Keutsch2,6, Jennifer Kaiser7,a, Kyung-Eun Min8,9,b, Steven S. Brown9,10, Rebecca A. Washenfelder8,9, Gonzalo González Abad11, and Kelly Chance11 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA 2School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA 3School of Chemistry and School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW, Australia 4Atmospheric Chemistry and Dynamics Lab, NASA Goddard Space Flight Center, Greenbelt, MD, USA 5Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD, USA 6Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA, USA 7Department of Chemistry, University of Wisconsin Madison, Madison, WI, USA 8Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA 9Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA 10Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USA 11Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA anow at: School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA bnow at: School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology, Gwangju, South Korea

Correspondence to: Daniel J. Jacob ([email protected])

Received: 23 November 2016 – Discussion started: 29 November 2016 Revised: 5 June 2017 – Accepted: 7 June 2017 – Published: 18 July 2017

Abstract. Glyoxal (CHOCHO) is produced in the atmo- Earth Observing System chemical transport model (GEOS- sphere by the oxidation of volatile organic compounds Chem) featuring a new chemical mechanism for CHOCHO (VOCs). Like formaldehyde (HCHO), another VOC oxida- formation from isoprene. The mechanism includes prompt tion product, it is measurable from space by solar backscat- CHOCHO formation under low-NOx conditions following ter. Isoprene emitted by vegetation is the dominant source the isomerization of the isoprene peroxy radical (ISOPO2). of CHOCHO and HCHO in most of the world. We use The SENEX observations provide support for this prompt aircraft observations of CHOCHO and HCHO from the CHOCHO formation pathway, and are generally consistent SENEX campaign over the southeast US in summer 2013 with the GEOS-Chem mechanism. Boundary layer CHO- to better understand the CHOCHO time-dependent yield CHO and HCHO are strongly correlated in the observations from isoprene oxidation, its dependence on nitrogen oxides and the model, with some departure under low-NOx condi- (NOx ≡ NO + NO2), the behavior of the CHOCHO–HCHO tions due to prompt CHOCHO formation. SENEX vertical relationship, the quality of OMI CHOCHO satellite observa- proﬁles indicate a free-tropospheric CHOCHO background tions, and the implications for using CHOCHO observations that is absent from the model. The OMI CHOCHO data pro- from space as constraints on isoprene emissions. We sim- vide some support for this free-tropospheric background and ulate the SENEX and OMI observations with the Goddard show southeast US enhancements consistent with the iso-

Published by Copernicus Publications on behalf of the European Geosciences Union. 8726 C. Chan Miller et al.: Glyoxal yield from isoprene prene source but a factor of 2 too low. Part of this OMI bias generate HOx radicals (Peeters et al., 2009, 2014; Crounse is due to excessive surface reflectivities assumed in the re- et al., 2011). trieval. The OMI CHOCHO and HCHO seasonal data over The fate of ISOPO2 determines the production rates and the southeast US are tightly correlated and provide redundant overall yields of CHOCHO and HCHO. Several studies have proxies of isoprene emissions. Higher temporal resolution provided insight on the time- and NOx-dependent yield of in future geostationary satellite observations may enable de- HCHO (Palmer et al., 2003; Marais et al., 2012; Wolfe et al., tection of the prompt CHOCHO production under low-NOx 2016). Under high-NOx conditions, HCHO production is conditions apparent in the SENEX data. sufficiently prompt that observed HCHO columns can be lo- cally related to isoprene emission rates (Palmer et al., 2006). This assumption is the basis of many studies that have used satellite HCHO observations to constrain isoprene emissions (Palmer et al., 2006; Fu et al., 2007; Millet et al., 2008; Curci 1 Introduction et al., 2010). HCHO production is much slower under low- NOx conditions, spatially “smearing” the local relationship Glyoxal (CHOCHO) and formaldehyde (HCHO) are short- between isoprene emissions and HCHO columns. This has lived products of the atmospheric oxidation of volatile or- been addressed by using concurrent satellite data for NO2 ganic compounds (VOCs). Both are detectable from space columns to correct the isoprene–HCHO relationship (Marais by solar backscatter (Chance et al., 2000; Wittrock et al., et al., 2012) or by using adjoint-based inverse modeling to 2006). Isoprene emitted by terrestrial vegetation accounts relate HCHO columns to isoprene emissions including the for about a third of the global source of non-methane VOCs effect of transport (Fortems-Cheiney et al., 2012). (NMVOCs Guenther et al., 2012) and drives large enhance- Isoprene is estimated to account for about ∼ 50 % of ments of CHOCHO and HCHO in the continental boundary global CHOCHO production (Fu et al., 2008), but there is layer (Palmer et al., 2003; Fu et al., 2008). Satellite obser- large uncertainty regarding the yield of CHOCHO from iso- vations of HCHO have been widely used as a proxy to esti- prene oxidation. Open fires and aromatic VOCs can also mate isoprene emissions (Abbot et al., 2003; Palmer et al., be major sources of CHOCHO (Volkamer et al., 2001; Fu 2006; Millet et al., 2008; Curci et al., 2010; Barkley et al., et al., 2008; Chan Miller et al., 2016). Several studies have 2013), but there are uncertainties related to the HCHO yield used the measured CHOCHO–HCHO concentration ratio from isoprene oxidation (Marais et al., 2012) and the role RGF = [CHOCHO]/[HCHO] as an indicator of the dominant of other NMVOCs as HCHO precursors (Fu et al., 2007). VOC precursors. Vrekoussis et al.(2010) found higher RGF CHOCHO observations from space could provide a comple- values (> 0.04) from GOME-2 satellite observations in re- mentary constraint (Vrekoussis et al., 2009, 2010; Alvarado gions where biogenic VOCs are dominant, and lower val- et al., 2014; Chan Miller et al., 2014). Here we use CHO- ues where anthropogenic VOCs are dominant. However, the CHO and HCHO aircraft observations over the southeast opposite behavior is observed in ground-based studies (Di- US from the summer 2013 Southeast Nexus (SENEX) cam- Gangi et al., 2012). Our recent CHOCHO retrieval from the paign (Warneke et al., 2016), interpreted with the Goddard OMI satellite instrument (Chan Miller et al., 2014) is in bet- Earth Observing System chemical transport model (GEOS- ter agreement with surface observations of CHOCHO and Chem), to test understanding of the CHOCHO yield from RGF (Kaiser et al., 2015) compared to those from GOME-2 isoprene oxidation, its dependence on nitrogen oxide rad- (Vrekoussis et al., 2010) and SCIAMACHY (Wittrock et al., icals (NOx ≡ NO + NO2), and the combined value of the 2006) as a result of improved background corrections and re- CHOCHO–HCHO pair measured from space to constrain moval of NO2 interferences. There remains the question of isoprene emissions and chemistry. how observed CHOCHO–HCHO relationships are to be in- Isoprene impacts air quality and climate as a precursor terpreted. to ozone (Geng et al., 2011) and secondary organic aerosol The Southeast Nexus (SENEX) aircraft campaign was (SOA Carlton et al., 2009), and also affects concentrations conducted over the southeast US in June–July 2013. The air- of hydrogen oxide radicals (HOx ≡ OH + HO2; Peeters and craft had a detailed chemical payload including in situ CHO- Muller, 2010) and NOx (Mao et al., 2013; Fisher et al., CHO (Min et al., 2016) and HCHO (Cazorla et al., 2015). 2016). Atmospheric oxidation of isoprene by OH takes place Thirteen daytime flights were conducted over the campaign on a timescale of less than an hour to produce organic per- with extensive boundary layer coverage. Li et al.(2016) oxy radicals (ISOPO2). Reaction of ISOPO2 with NO drives recently used the SENEX observations to evaluate CHO- production of ozone and of organic nitrates that serve as CHO formation from isoprene in the AM3 chemical trans- a reservoir for NOx (Browne and Cohen, 2012). At lower port model (CTM). They found that the AM3 mechanism NOx levels, ISOPO2 reacts dominantly with HO2 to pro- had closer agreement with observations than the explicit duce isoprene epoxydiols (IEPOX) via isoprene peroxides Master Chemical Mechanism v3.3.1 (MCMv3.3.1; Jenkin (ISOPOOH; Paulot et al., 2009b), and from there isoprene et al., 2015), and suggested that CHOCHO yields from iso- SOA (Marais et al., 2016). ISOPO2 can also isomerize to prene epoxydiols are underestimated in MCMv3.3.1. Here

Atmos. Chem. Phys., 17, 8725–8738, 2017 www.atmos-chem-phys.net/17/8725/2017/ C. Chan Miller et al.: Glyoxal yield from isoprene 8727 we take a more rigorous look at potential missing pathways to the terminal carbons of the isoprene double bonds (po- in MCMv3.3.1. In doing so, we present an improved chem- sitions 1 and 4, Fig.1). Isoprene peroxy radicals (ISOPO 2) ical mechanism for CHOCHO formation from isoprene for are formed by O2 addition to the carbon either in β or δ to the GEOS-Chem CTM, and evaluate it against the SENEX the hydroxyl carbon. ISOPO2 reacts with NO and HO2, and observations, including the time and NOx dependence of the also isomerizes. Together these pathways represent 92 % of CHOCHO yield from isoprene. We discuss the implications ISOPO2 loss, with the remainder due to reactions with or- of the new mechanism for the interpretation of satellite ob- ganic peroxy radicals. servations, and present a first validation of the CHOCHO re- Under high-NOx conditions, CHOCHO is produced trieval from the OMI satellite instrument (Chan Miller et al., promptly via products of the δ isomers (HC5, DIBOO; Paulot 2014). et al., 2009a; Galloway et al., 2011). CHOCHO production via the β isomers is slower, due to the intermediary production of methylvinylketone (MVK) followed by glycolalde- 2 GEOS-Chem model description hyde (GLYC). GEOS-Chem originally had a fixed δ vs. β branching ratio of 24 % for the reaction of ISOPO + NO, 2.1 General description 2 based on the chamber experiments of Paulot et al.(2009a). We use the same version of GEOS-Chem v9.2 (http://www. However recent work has shown that O2 addition to the geos-chem.org) that has been used previously to interpret isoprene–OH adducts is reversible (pink pathway, Fig.1), chemical observations from the NASA SEAC4RS aircraft allowing interconversion between β and δ ISOPO2 isomers campaign conducted in the same southeast US region in (Peeters et al., 2009, 2014; Crounse et al., 2011). Isomers of ∼ August–September 2013 (Travis et al., 2016; Fisher et al., β are heavily favored at equilibrium, accounting for 95 % 2016). The model is driven by assimilated meteorologi- of ISOPO2 (Peeters et al., 2014). The experimental condi- cal data with 0.25◦ × 0.3125◦ horizontal resolution from tions in Paulot et al.(2009a) used high NO x concentrations ∼ the Goddard Earth Observing System (GEOS-FP) reanaly- ( 500 ppbv). This implies short ISOPO2 lifetimes, and thus sis product (Molod et al., 2012). The native 0.25◦ × 0.3125◦ may not reflect the degree of isomer interconversion seen at resolution is retained in GEOS-Chem over the North Ameri- ambient oxidant levels. Here we adopt a δ-ISOPO2 branch- can domain (130–60◦ W, 9.75–60◦ N), nested within a global ing ratio of 10 %, following Fisher et al.(2016), to match 4 simulation at 2◦ × 2.5◦ resolution (Kim et al., 2015). Iso- SEAC RS observations of organic nitrates produced through + prene chemistry in GEOS-Chem v9.2 is as described by Mao the δ-ISOPO2 NO pathway. et al.(2013), but the SEAC 4RS simulation includes a number CHOCHO forms under low-NOx conditions through iso- of updates described by Travis et al.(2016) and Fisher et al. prene epoxydiols (IEPOX) and through the ISOPO2 iso- (2016). The simulation presented here includes further modi- merization pathway. IEPOX forms as a second-generation fications relevant to CHOCHO, listed in the Supplement (Ta- non-radical product of isoprene oxidation via ISOPOOH, ble S1) and summarized below. Evaluation of the model with and thus represents a slow CHOCHO formation pathway. SEAC4RS observations has been presented by Kim et al. IEPOX isomer fractions in GEOS-Chem are based on equilibrium δ / β ISOPO2 branching ratios (Bates et al., 2014; (2015) for aerosols, Travis et al.(2016) for ozone and NO x, Fisher et al.(2016) for organic nitrates, Marais et al.(2016) Travis et al., 2016). At low NOx levels the ISOPO2 lifetime for isoprene SOA, and Zhu et al.(2016) for HCHO including is sufficiently long for equilibrium to be reached (Peeters satellite validation. et al., 2014). ISOPO2 isomerization in the previous GEOS- Isoprene emissions in the model are from MEGANv2.1 Chem mechanism of Travis et al.(2016) produced solely (Guenther et al., 2012) with a 15 % reduction (Kim et al., hydroperoxyaldehydes (HPALDs), but here we also include the formation of dihydroperoxy α-formyl peroxy radicals 2015), and NOx emissions are as described by Travis et al. (2016) including a 50 % decrease in the anthropogenic source (di-HPCARPs; Peeters et al., 2014) following MCMv3.3.1. relative to the 2011 National Emission Inventory of the US di-HPCARPs in MCMv3.3.1 have a low CHOCHO yield, Environmental Protection Agency. Yu et al.(2016) pointed but here we introduce a (1,5)H-shift isomerization of di- HPCARPs that could be competitive with the (1,4)H-shift out that isoprene and NOx emissions in the southeast US are spatially segregated and show that the 0.25◦ × 0.3125◦ reso- isomerization due to the presence of the terminal-peroxide lution of GEOS-Chem is adequate for separating the popu- functional group (Crounse et al., 2013). The resulting di-hydroperoxide dicarbonyl compound (DHDC) product lations of high- and low-NOx conditions for isoprene oxidation. quickly photolyzes to produce CHOCHO, analogous to the mechanisms proposed for HPALDs (Peeters et al., 2014) and 2.2 CHOCHO formation from isoprene and loss carbonyl nitrates (Müller et al., 2014). As shown below, we find that this pathway can explain SENEX observations of Figure1 shows the CHOCHO formation pathways from iso- prompt CHOCHO production under low-NOx conditions. prene oxidation by OH (the main isoprene sink) as imple- The mechanism presented here differs substantially from mented in this work. Oxidation is initiated by OH addition the AM3 mechanism previously used by Li et al.(2016) www.atmos-chem-phys.net/17/8725/2017/ Atmos. Chem. Phys., 17, 8725–8738, 2017 8728 C. Chan Miller et al.: Glyoxal yield from isoprene

3 Mean branching ratios 1 2 Isoprene 4 Contributions to CHOCHO formation Contributions to glycolaldehyde (GLYC) formation Lifetime OH

OH HO (Major isomers) 1-OH 4-OH

O2 -O2

OH OO OO β δ OO OH OH HO OO ISOPO2 Z-1-OH,4-O Z-4-OH,1-O 1-OH,2-O 4-OH,3-O 2 2 2 2 (& E isomers)

(1,6)H 48 % Isom. 10 % HO2 34 % NO 50 % 50 % (1,4)H δ Isom. O O β (90 %) (10 %) OO (1,5)H OOH O OH Isom. OOH OH OH O ISOPOOH HO MVK HC5 O OOH OOH 0.9 h 3.5 h DIBOO O di-HPCARP HPALD 0.6 h OOH 0.7 h OH OH NO OH NO/HO2 O hν 2 % 66 % DHDC OH O OOH 0.7 h HO O 25 % 4 % OH OH 3 % IEPOX GLYC hν 5.5 h 8.7 h OH OH 26 % 17 % 13 % 7 % O O 1 % 9 % 26 % CHOCHO

Figure 1. Pathways for glyoxal (CHOCHO) formation from isoprene oxidation in GEOS-Chem as implemented in this work. Only species relevant to CHOCHO formation are shown. Branching ratios, species lifetimes, and contributions to glyoxal and glycolaldehyde (GLYC) formation from each boxed species are mean values over the southeast US (96.25–73.75◦ W, 29–41◦ N) during the SENEX campaign (1 June– 10 July 2013). Species lifetimes are shown for an OH concentration of 4 × 106 molecules cm−3. to analyze the SENEX observations. Li et al.(2016) tested 2013; Marais et al., 2016). However HPALD photolysis is branching ratios of 22 and 0 % for δ-ISOPO2 + NO, with not expected to yield CHOCHO (Sect. S3). The CHOCHO the latter intended to reflect ISOPO2 isomer interconversion. formation pathway via DHDC proposed here can be justified The 10 % branching ratio in this study is constrained by from existing literature (Sect. S3). Inclusion of DHDC in- 4 SEAC RS organic nitrate observations (Fisher et al., 2016). creases the yield of CHOCHO via ISOPO2 isomerization by Li et al.(2016) report a CHOCHO yield from GLYC oxida- 18 %, which is comparable to the AM3 yield. tion (Sect. S1 in the Supplement), which is mainly due to a Li et al.(2016) found that CHOCHO concentrations are lower CHOCHO yield from GLYC + OH (13 % vs. 20 %). sensitive to aerosol reactive uptake. Our standard model sim- Their yield of CHOCHO from IEPOX is 28 %, much higher ulation does not include this uptake, but we conducted a than can be accommodated by yields of hydroxyacetone de- sensitivity simulation with a reactive uptake coefficient γ = rived from IEPOX oxidation chamber experiments (Bates 2 × 10−3 from Li et al.(2016). We find that CHOCHO con- et al., 2014) (the expected coproduct of CHOCHO via this centrations decrease by only 10 % on average (Sect. S4) be- pathway, Sect. S2). Following Travis et al.(2016), we set the cause competing CHOCHO sinks from reaction with OH and CHOCHO yield from IEPOX to the corresponding hydroxy- photolysis are fast. acetone yields reported by Bates et al.(2014) (8.5 % via HO 2 and 8.8 % via NO). Finally AM3 assumes 25 % CHOCHO yield from HPALD photolysis following Stavrakou et al. (2010), which has been used in many past studies (Mao et al.,

Atmos. Chem. Phys., 17, 8725–8738, 2017 www.atmos-chem-phys.net/17/8725/2017/ C. Chan Miller et al.: Glyoxal yield from isoprene 8729

(a) ISOPO2 f ate (b) CHOCHO y ie ld (c) HCHO y ie ld GEOS-Chem 1.0 1.0 0.03 1.0 1.5 MCMv3.3.1 0.6 1.5 0.05 0.6 0.9 0.5 0.5 0.05 0.5 0.9 1.2 (ppbv) x

NO 0.9 0.3 0.1 NO 0.1 0.01 0.1 0.9 HO 0.03 0.3 2 0.05 0.6 0.03 0.07 0.6 Isom. 0.01 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 0 2 4 6 8 10 Molar y ield OH exposure t ime (h) OH exposure t ime (h)

Figure 2. Cumulative time- and NOx-dependent molar yields of CHOCHO and HCHO from isoprene oxidation in the GEOS-Chem and MCM3.3.1 chemical mechanisms. Results are from box model simulations with ﬁxed NOx concentrations as described in the text, and are presented as functions of the imposed NOx concentration (vertical axis). Panel (a) shows the isoprene peroxy radical (ISOPO2) branching ratios for reaction with NO, HO2, and isomerization. Panels (b, c) show the time-dependent cumulative yields of CHOCHO and HCHO, where time is normalized by OH exposure (Eq.1). “OH exposure time” is equivalent to time for a constant [OH] = 4 × 106 molecules cm−3.

2.3 Time- and NOx-dependent CHOCHO and HCHO the observed correlation between HCHO and isoprene or- yields from isoprene ganic nitrates (Mao et al., 2013; Fisher et al., 2016). The HCHO yield is lower under low-NOx conditions in both Understanding the time- and NOx-dependent yields of CHO- mechanisms, and overall the difference between them is mi- CHO and HCHO from isoprene oxidation is critical for in- nor. terpreting observed CHOCHO and HCHO columns from There is far more disagreement between the two mech- space in terms of isoprene emissions. Here we examine anisms for CHOCHO yields. Under high-NOx conditions, time-dependent CHOCHO and HCHO molar yields in the GEOS-Chem produces CHOCHO rapidly in the first 2 h due GEOS-Chem and MCMv3.3.1 chemical mechanisms using to its higher δ-ISOPO2+NO branching ratio (10 % in GEOS- the Dynamically Simple Model of Atmospheric Chemical Chem vs. 3.4 % in MCMv3.3.1). This is compensated at Complexity (DSMACC) box model (Emmerson and Evans, longer OH exposure times by higher GLYC yields from iso- 2009). Simulations are initiated at 09:00 LT with 1 ppbv isoprene in MCMv3.3.1. GEOS-Chem produces higher ultimate prene, 40 ppbv O3, and 100 ppbv CO. NOx concentrations yields of CHOCHO under low-NOx conditions mainly due are held at fixed values. Photolysis rates are calculated for to DHDC formation and subsequent photolysis, neither of clear sky with the TUV radiative transfer model (Madronich, which are included in MCMv3.3.1. The NOx-dependence 1987). To correct for differences in time-dependent yields as- of the CHOCHO yield in MCMv3.3.1 is similar to that of sociated with differences in OH concentrations, we reference HCHO, implying that CHOCHO and HCHO observations GEOS-Chem and MCMv3.3.1 results to a common “OH ex- would provide redundant information on isoprene emissions. posure time” variable (tOH): The SENEX observations indicate that CHOCHO yields under low-NOx conditions are too low in MCMv3.3.1, as dis- t Z cussed below. In GEOS-Chem, by contrast, the CHOCHO 1 0 0 tOH = [OH](t )dt . (1) and HCHO yields show opposite dependences on NOx, im- [OH]ref 0 plying that they could provide complementary information on isoprene emissions. Here [OH](t) is the OH concentration simulated in the 6 −3 box model, and [OH]ref = 4 × 10 molecules cm is a reference OH concentration representative of summer daytime 3 Constraints from SENEX observations conditions over the southeast US (Wolfe et al., 2016). For a 6 −3 fixed [OH] = 4 × 10 molecules cm , tOH represents the ac- Figure3 shows the observed and simulated median verti- tual time. cal profiles of CHOCHO, HCHO, and NOx concentrations Figure2 shows the time- and NO x-dependent cumulative along the SENEX flight tracks. Figure4 shows maps of con- molar yields of CHOCHO and HCHO in GEOS-Chem and centrations below 1 km altitude (above ground level) taken MCMv3.3.1. The branching ratio of ISOPO2 as a function as the mixed layer. Here and elsewhere we only include of NOx is also shown. The time-dependent HCHO yields daytime observations (10:00–17:00 LT) and exclude targeted in both mechanisms are similar under high-NOx conditions. sampling of biomass burning plumes (diagnosed by ace- Additional confidence in the HCHO yield under these condi- tonitrile concentrations above 200 pptv). CHOCHO, HCHO, tions is offered by the ability of GEOS-Chem to reproduce and NOx were measured by the Airborne Cavity Enhanced www.atmos-chem-phys.net/17/8725/2017/ Atmos. Chem. Phys., 17, 8725–8738, 2017 8730 C. Chan Miller et al.: Glyoxal yield from isoprene

CHOCHO HCHO NOx 5 5 5

4 4 4 Observed GEOS-Chem 3 3 3

2 2 2 Altitude (km) 1 1 1

0 0 0 0 50 100 150 0 1 2 3 4 5 6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Concentration (pptv) Concentration (ppbv) Concentration (ppbv)

Figure 3. Median vertical proﬁles of CHOCHO, HCHO, and NOx concentrations during SENEX (1 June–10 July 2013). Observed concentrations (Min et al., 2016; Cazorla et al., 2015; Pollack et al., 2010) are compared to GEOS-Chem model values sampled along the ﬂight tracks. Horizontal bars indicate interquartile range. Altitudes are above ground level (a.g.l.).

Spectrometer (ACES; Min et al., 2016), In Situ Airborne The mixed layer concentrations maps in Fig.4 show that Formaldehyde (ISAF) instrument (Cazorla et al., 2015), and the model captures some of the horizontal variability in the NOAA chemiluminescence instrument (Ryerson et al., the observations. The spatial correlation for HCHO is high 1999; Pollack et al., 2010), with stated accuracies of 6, 10, (r = 0.75) as in SEAC4RS (r = 0.64, Zhu et al., 2016), and and 5 %, respectively. reflects isoprene emission patterns. Correlation for CHO- Simulated median NOx concentrations in the mixed layer CHO is also relatively strong (r = 0.51). Temporally aver- are within 10 % of observations, supporting the 50 % reduc- aged CHOCHO and HCHO concentrations simulated by the tion in EPA NEI NOx emissions previously inferred from the model for the SENEX period (background in Fig.4) are analysis of SEAC4RS observations by Travis et al.(2016), much more uniform than those sampled along the SENEX also included here (Sect. 2.1). Half of isoprene oxidation in flight tracks because of day-to-day variability in isoprene the model under the SENEX conditions takes place by the emissions, mostly driven by temperature (Zhu et al., 2016). low-NOx pathways (Fig.1). Simulated median CHOCHO Figure5 compares simulated and observed CHOCHO vs. and HCHO concentrations in the mixed layer are within 20 % HCHO relationships in the mixed layer, color coded by of observations, but the model is too low at higher altitudes. NOx concentrations. Correlation between the two species is During SENEX the mixed layer was typically capped by a strong. The model better captures the observed slope (0.028 neutrally stable transition layer of shallow cumulus convec- modeled vs. 0.024 observed) compared to the AM3 CTM tion which extended up to 3 km (Wagner et al., 2015), which (0.045 and 0.035 with and without CHOCHO production could suggest that the model underestimates transport via from δ-ISOPO2 + NO, respectively; Li et al., 2016). Inclu- this mechanism. However, the model does not underestimate sion of aerosol uptake further reduces the bias to the observed other isoprene oxidation products in the transition layer, such slope (0.026, Fig. S10). On average, CHOCHO is produced as MVK + methacrolein (Fig. S8 in the Supplement). An- more promptly in AM3 compared to GEOS-Chem, which other possible source of CHOCHO in the transition layer is may lead to the higher slope. In the first few hours of oxi- via heterogeneous aerosol oxidation (Volkamer et al., 2015). dation this is due to a higher CHOCHO yield from ISOPO2 However, specific aerosol precursors that produce CHOCHO isomerization. Beyond the initial stages of isoprene oxida- at yields required to match the SENEX observations are cur- tion, CHOCHO is produced faster in AM3 because of the rently unknown (Kaiser et al., 2015). increased fraction of CHOCHO produced from IEPOX over The CHOCHO observations in the free troposphere GLYC oxidation (Fig.1). (> 3 km) have to be treated with caution since they are be- The strong correlation between CHOCHO and HCHO low the reported instrument precision (32 pptv, Kaiser et al., might suggest that they provide redundant information for 2015). It is therefore difficult to determine whether the bias is constraining isoprene emissions. However, examination of due to a missing CHOCHO source in the model or instrument Fig.5 indicates higher observed CHOCHO-to-HCHO ratios artifact. Elevated CHOCHO concentrations above the bound- (RGF) at low-NOx concentrations, not captured by GEOS- ary layer have also been observed in previous campaigns over Chem. Figure6 shows the RGF ratio as a function of the southeast US (Lee et al., 1998), California (Baidar et al., NOx below 1 km in the SENEX observations and as sim- 2013), and the remote Pacific (Volkamer et al., 2015). There ulated by GEOS-Chem. Points are color coded by OH ex- could be a free-tropospheric source missing in the model, posure time tOH (Eq.1), derived from PTR-MS observa- but it is unclear what this source could be, and correlative tions of the methylvinylketone + methacrolein-to-isoprene analysis of observed free-tropospheric CHOCHO with other ratio (de Gouw and Warneke, 2007) following Wolfe et al. species measured in SENEX offer no insight (r < 0.3 for all (2016). The median and interquartile RGF values binned in observed VOCs). 250 pptv NOx increments are also shown. The observed me-

Atmos. Chem. Phys., 17, 8725–8738, 2017 www.atmos-chem-phys.net/17/8725/2017/ C. Chan Miller et al.: Glyoxal yield from isoprene 8731

low CHOCHO values in the model that are not found in Concentrations below 1 km altitude the observations where the concentration ﬂoor is 0.05 ppbv Observed GEOS-Chem ≥0.16 (Fig.5). There may be a CHOCHO background miss-

o 40 N 0.12 ing from the model, possibly contributed by monoterpenes; MCMv3.3.1 predicts that the total CHOCHO yield from 0.08 35o N common monoterpenes is high (Kaiser et al., 2015), and that r = 0.51 0.04 o they produce CHOCHO over a timescale of days (Fig. S11). 30 N NMB = -17% CHOCHO CHOCHO 0.00 ≥7.00

o 40 N 5.25 4 Implications for satellite observations

o 3.50 35 N Knowledge gained from SENEX enables an improved in- r = 0.75 1.75 30o N terpretation of CHOCHO and HCHO column observations HCHO HCHO NMB = -17% 0.00 from space in isoprene dominated environments. We use for Concentration (ppbv) ≥1.00 this purpose June–August in 2006 and 2007 observations of

o 40 N 0.75 CHOCHO, HCHO, and tropospheric NO2 columns from the Ozone Monitoring Instrument (OMI). OMI was launched on- o 0.50 35 N board the NASA Aura satellite in July 2004, and provides r = 0.54 0.25 o daily global coverage in sun-synchronous orbit with an equa- 30 N NMB = -13% NOx NOx 0.00 torial crossing time of 13:40 LT. The CHOCHO data are from 95o W 90o W 85o W 80o W 7 5 o W 95o W 90o W 85o W 80o W 75o W the Smithsonian Astrophysical Observatory (SAO) retrieval Figure 4. CHOCHO, HCHO, and NOx concentrations below described in Chan Miller et al.(2014) and hereby referred to 1 km a.g.l. during SENEX (1 June–10 July 2013). The grid squares as OMI SAO. The HCHO and NO2 data are from the OMI show daytime aircraft observations compared to the colocated Version 3 product release (González Abad et al., 2015; Buc- ◦ ◦ GEOS-Chem model values on the 0.25 × 0.3125 model grid. sela et al., 2013). Retrievals are in the 435–461 nm spectral Background contours in the right panels show the average model- range for CHOCHO, 328.5–356.5 nm for HCHO, and 405– simulated concentrations at 13:00–14:00 LT for the SENEX period. 465 nm for NO2. We use 2006–2007 data because 2013 data Comparison statistics between model and observation grid squares for CHOCHO are very noisy (Fig. S12), possibly because are shown as the correlation coefficient r and the normalized mean bias (NMB). Correlation statistics for NO exclude urban plumes in of sensor degradation. The OMI observations are compared 2 to a GEOS-Chem simulation covering the same period, at the observations ([NOx] > 4 ppb) as these would not be resolved at ◦ ◦ the scale of the model. 2 × 2.5 horizontal resolution. Slant columns along the optical path of the backscattered solar radiation are fitted to the observed spectra and con- −1 dian RGF values (0.02 to 0.024 mol mol ) show no signifi- verted to vertical columns by division with an air mass factor cant dependence on NOx, while GEOS-Chem shows a weak (AMF) that accounts for the viewing geometry, atmospheric dependence. scattering, and the vertical profile of the gas (Palmer et al., The observations contain a subset of low-NOx points with 2001): higher RGF values (0.03–0.06). The model also produces a subset of enhanced RGF values under low-NOx conditions, ∞ although peak RGF values are lower than the observations. Z In both cases, the enhanced RGF values coincide with short AMF = w(z)s(z)dz. (2) OH exposure times, which are caused by OH titration by iso- 0 prene. The high RGF reflects the relatively faster production of CHOCHO than HCHO in the early stage of isoprene ox- Here w(z) is the scattering weight measuring the sensitiv- idation under low-NOx conditions as shown by Fig.2. The ity of the retrieval to the gas concentration at altitude z, and presence of that population in the observations provides sup- s(z) is a normalized vertical profile of gas number density. port for fast glyoxal production from the isomerization path- Here we recomputed the AMFs for the three retrievals us- way of isoprene oxidation (Fig.1) that is present in GEOS- ing vertical profiles from GEOS-Chem, as it is necessary for Chem but not in MCMv3.3.1. The model may not capture comparing simulated and observed vertical columns (Duncan the highest observed RGF values due to uncertainties in the et al., 2014). yield of DHDC from isoprene and its photolysis rate, both We remove observations impacted by the row of which have been estimated based on literature proxies anomaly (http://www.knmi.nl/omi/research/product/ (Sect. S3). rowanomaly-background.php), and those with cloud Figure6 also shows that there is a small subset of points fractions over 20 %. Previous validation of the OMI HCHO in GEOS-Chem with RGF values less than 0.01, reflecting retrievals with SEAC4RS aircraft observations revealed a www.atmos-chem-phys.net/17/8725/2017/ Atmos. Chem. Phys., 17, 8725–8738, 2017 8732 C. Chan Miller et al.: Glyoxal yield from isoprene

0.85

0.30 r = 0.73 0.30 r = 0.92 Slope = 0.024 Slope = 0.028 0.25 0.25 0.65 Intercept = -9 pptv Intercept = -22 pptv 0.20 0.20 0.45 0.15 (ppbv) 0.15 x

0.10 0.10 NO 0.25

CHOCHO (ppbv) CHOCHO (ppbv) 0.05 0.05 Observed GEOS-Chem 0.00 0.00 0.05 0 2 4 6 8 10 0 2 4 6 8 10 HCHO (ppbv) HCHO (ppbv)

Figure 5. Relationship between CHOCHO and HCHO concentrations in the mixed layer (< 1 km a.g.l.) during SENEX (1 June– 10 July 2013), color coded by NOx concentration. Observed concentrations (Min et al., 2016; Cazorla et al., 2015) are compared to GEOS- Chem model values sampled along the ﬂight tracks. Lines and reported slopes are from reduced major axis regressions. )

-1 (a) l (b) 1.50 o 0.06 0.06 m

o Observed GEOS-Chem

m 0.05 0.05 1.25 (

] O

H 0.04 0.04 1.00

C H [

/ 0.03 0.03 ] 0.75

H 0.02 0.02 C

O 0.50 H

0.01 0.01 OH exposuret ime (h) C [

= 0.00 0.00 0.25 F

G 0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.5 1.0 1.5 2.0 2.5 R NOx (ppbv) NOx (ppbv)

Figure 6. Dependence of the CHOCHO-to-HCHO ratio RGF on NOx concentrations for the SENEX conditions. Observations below 1 km altitude (a) are compared to GEOS-Chem model values sampled along the ﬂight tracks (b). Points are color coded by the OH exposure time tOH (Eq.1). Binned median and interquartile RGF values in increments of 250 pptv NOx for bins with more than 20 values are also shown.

43 % uniform low bias (Zhu et al., 2016), corrected in the similar wavelengths so the sensitivity to surface reflectivity data shown here. should be similar. Figure7 (bottom right) shows the mean Figure7 compares CHOCHO and HCHO vertical columns CHOCHO scattering weights computed from the OMI-SAO from GEOS-Chem and OMI, and Fig.8 shows spatial corre- and BEHR. The lower BEHR surface reflectivity values re- lations over the eastern US. Excellent agreement is found for sult in a lower AMF and hence a higher vertical column HCHO, providing an independent test of the correction to (Fig.7, bottom left panel). The slope of the regression be- the OMI HCHO retrieval inferred from the SEAC4RS data tween GEOS-Chem and OMI CHOCHO columns increases (Zhu et al., 2016). Since GEOS-Chem can also replicate the from 0.48 to 0.62, improving but not reconciling the differ- CHOCHO–HCHO correlation in the SENEX data, the sim- ences. ulated CHOCHO columns can be used to indirectly validate As pointed out above, SENEX and other observations the OMI CHOCHO observations. CHOCHO from OMI is suggest that GEOS-Chem may be missing a background highly correlated with GEOS-Chem (r = 0.81), indicative source of CHOCHO. Integration of the median CHOCHO of the isoprene source. However OMI CHOCHO shows a profile above 2 km in Fig.3 shows a negative model bias higher continental background and a factor of 2 weaker en- of 1.3 × 1014 molecules cm−2, comparable to the continental hancement over the southeast US. background intercept in Fig.8 (1 .9 × 1014 molecules cm−2). Zhu et al.(2016) suggested that errors in the assumed The nonzero intercept may in part reflect an underesti- surface reflectivities affecting the AMFs were an important mate of CHOCHO concentrations caused by a missing source of the bias in the OMI HCHO retrievals. CHOCHO CHOCHO source over the southeast US, such as monoter- retrievals are even more sensitive to surface reflectivity be- penes (Sect.3). The presence of free-tropospheric CHOCHO cause of the longer wavelengths. Russell et al.(2011) previ- would further impact the AMF calculation under continen- ously pointed out that the OMI surface reflectivities used in tal background conditions since the retrieval sensitivity as the standard NO2 retrievals (Kleipool et al., 2008) were too measured by the scattering weights increases with altitude. high and replaced them with high-resolution (0.05◦ × 0.05◦) Thus the retrieved continental background would be overes- reflectivity observations from MODIS (Schaaf and Wang, timated. 2015) to produce the Berkeley High-Resolution (BEHR) Figure9 shows CHOCHO vs. HCHO relationships for OMI NO2 retrieval. CHOCHO and NO2 are retrieved at OMI (using the BEHR scattering weights) and GEOS-Chem,

Atmos. Chem. Phys., 17, 8725–8738, 2017 www.atmos-chem-phys.net/17/8725/2017/ C. Chan Miller et al.: Glyoxal yield from isoprene 8733

(a)

45o N 45o N

40o N 40o N -2 -2

35o N ≥ 0.50 35o N ≥ 24

30o N 30o N SAO: 28 % 0.37 SAO: -7 % 19

GEOS-Chem CHOCHO HCHO 25o N BEHR: -4 % 25o N 100o W 95o W 90o W 85o W 80o W 75o W 70o W 100o W 95o W 90o W 85o W 80o W 75o W 70o W 15 0.25 15 14 (b) 45o N 45o N 0.12 9

40o N 40o N 0.00 4 35o N 35o N Column density (10density Column cmmolecules ) Column density (10density Column cmmolecules )

30o N 30o N

OMI-SAO 0.50

CHOCHO o HCHO 25o N 25 N o o o o o o o 100o W 95o W 90o W 85o W 80o W 75o W 70o W 100 W 95 W 90 W 85 W 80 W 75 W 70 W 0.37 (c) CHOCHO scattering w eights 200 45o N SENEX 400 GEOS-Chem 0.25 40o N OMI-SAO 600 OMI-BEHR 35o N 800 Pressure (hPa) 0.12 30o N 1000

OMI-BEHR 0 2 4 6 CHOCHO Scattering Weights, Shape Factors 25o N Scattering w eights (solid) Shape f actors (dashed) 0.00 100o W 95o W 90o W 85o W 80o W 75o W 70o W

Figure 7. Mean CHOCHO and HCHO columns in summer (JJA) 2006–2007. GEOS-Chem model values (a) are compared to OMI satellite observations (b, c). OMI-SAO is the standard operational product (Chan Miller et al., 2014; González Abad et al., 2015). The OMI-BEHR product for CHOCHO uses tropospheric scattering weights from the BEHR NO2 retrieval (Russell et al., 2011; Laughner et al., 2016). The OMI HCHO observations have been scaled up by a factor of 1.67 to correct for retrieval bias (Zhu et al., 2016). The normalized mean bias (NMB) between GEOS-Chem and OMI in the southeast US (75–100◦ W, 29.5–37.5◦ N) is shown within the GEOS-Chem panels. The right panel of (c) shows the mean CHOCHO scattering weights (w) from the OMI-SAO and OMI-BEHR retrievals and the vertical shape factors (s) over the southeast US from the SENEX observations and GEOS-Chem in the southeast US from a typical orbit (10114, 9 June 2006).

CHOCHO HCHO color coded by tropospheric NO2 columns. Individual points

) 8 ) 4

-2 OMI-BEHR -2 are seasonal averages (data points from Fig.7) in order to r = 0.81 6 Slope = 0.62 ± 0.04 3 limit noise. The slope is steeper in GEOS-Chem because the CHOCHO columns are higher. Since GEOS-Chem re- 4 2 produces the aircraft CHOCHO–HCHO relationship with- molecules cm molecules cm

14 16 out bias (Fig.5), this is further evidence of bias in the 2 OMI-SAO 1 OMI-SAO r = 0.81 r = 0.95 OMI CHOCHO observations. The CHOCHO–HCHO rela-

OMI (10 0 Slope = 0.48 ± 0.04 OMI (10 0 Slope = 0.98 ± 0.03 tionship is tight in both OMI (r = 0.86) and GEOS-Chem 0 2 4 6 8 0 1 2 3 4 = GEOS-Chem (1014 molecules cm-2) GEOS-Chem (1016 molecules cm-2) (r 0.99), with no indication of a separate population of low-NOx points with high RGF as there was in the SENEX data. It thus appears from the OMI data that satellite obser- Figure 8. Scatterplots of OMI vs. GEOS-Chem CHOCHO and ◦ ◦ vations of CHOCHO and HCHO in isoprene-dominated en- HCHO columns over the eastern US (75–100 W, 29.5–45 N). Val- vironments are redundant. This may reflect the higher NO ues are seasonal means for JJA 2006–2007 as plotted in Fig.7. OMI x levels in 2006–2007 compared to 2013 (Russell et al., 2012). observations for CHOCHO are from the standard SAO retrieval (Chan Miller et al., 2014) and using BEHR scattering weights (Rus- However since median RGF shows no significant variation sell et al., 2011; Laughner et al., 2016). Correlation coefficients and with NOx in the SENEX data (Fig.6), the required temporal reduced-major-axis (RMA) regressions are shown. averaging of satellite observations is a more likely explanation for the tight correlation. Finer-scale and more temporally resolved data, as will be available from the TEMPO geostationary instrument to be launched in the 2018–2020 time www.atmos-chem-phys.net/17/8725/2017/ Atmos. Chem. Phys., 17, 8725–8738, 2017 8734 C. Chan Miller et al.: Glyoxal yield from isoprene

Relationship b etween CHOCHO and HCHO cc olumns cept at very low NOx where the [CHOCHO] / [HCHO] ratio ) )

-2 -2 3.00 0.6 0.6 ) (RGF) can be unusually high. This reﬂects prompt formation

-2 0.5 OMI-BEHR 0.5 GEOS-Chem 2 2.50 of CHOCHO under low-NOx conditions, which was miss- 0.4 0.4 ing from MCMv3.3.1 and is now simulated in our updated

molecules cm 0.3 molecules cm 0.3 2.00 15 15 GEOS-Chem mechanism by DHDC photolysis. A previous 0.2 0.2 molecules cm 1.50 r = 0.86 r = 0.99 15 model comparison to SENEX showed that MCMv3.3.1 un-

0.1 0.1 NO Tropospheric

slope = 0.018 slope = 0.028 (10 0.0 0.0 1.00 derestimates the CHOCHO yield from isoprene (Li et al.,

CHOCHO (10 0 5 10 15 20 25 30 CHOCHO (10 0 5 10 15 20 25 30 HCHO (1015 molecules cm-2) HCHO (1015 molecules cm-2) 2016). Our work shows the missing DHDC production pathway can explain approximately 60 % of this underestimate, Figure 9. Relationship between CHOCHO and HCHO vertical with the remainder caused by an underestimate of the δ- ◦ ◦ columns over the eastern US (75–100 W, 29.5–45 N) in June– ISOPO2 branching ratio (3.4 % in MCMv3.3.1 vs. 10 % in August 2006 and 2007 color coded by tropospheric NO2 columns. GEOS-Chem). OMI values with CHOCHO AMFs computed from BEHR scatter- The SENEX observations enable indirect validation of the ing weights are compared to GEOS-Chem values. Lines and re- OMI CHOCHO satellite data using GEOS-Chem as an in- ported slopes are from reduced major axis regressions. tercomparison platform. The OMI data show a continental background that is consistent with the SENEX free- frame (Zoogman et al., 2016), may provide new perspectives tropospheric observations, and an enhancement over the of the utility of the CHOCHO retrieval. southeast US that is consistent with the isoprene source. However this enhancement is a factor of 2 too low in the OMI data. A partial explanation is that surface reflectivi- 5 Conclusions ties assumed in the standard OMI retrieval are too high. The satellite data show strong CHOCHO–HCHO correlation con- We have used aircraft observations of glyoxal (CHOCHO), sistent with the model and imply that the two gases pro- formaldehyde (HCHO), and related species from the SENEX vide redundant information for constraining isoprene emis- aircraft campaign over the southeast US together with OMI sions in regions where isoprene is their dominant precursor. satellite data to better understand the CHOCHO yield from This redundancy may reflect the seasonal averaging in the isoprene and the complementarity of CHOCHO and HCHO OMI data required to reduce noise. Recent validation of the observations from space for constraining isoprene emissions. HCHO satellite data revealed negative retrieval biases (Zhu This work includes a first validation of the CHOCHO re- et al., 2016), which can be corrected using spatially uniform trieval from the OMI satellite instrument. scaling factors (as done in this study). Since similar biases We began with an analysis of the time- and NOx- may exist for the CHOCHO retrieval, the scaled HCHO data dependent CHOCHO and HCHO yields from isoprene ox- should at present be preferentially used as proxy for isoprene idation in the GEOS-Chem chemical transport model and in emissions. Future geostationary observations from TEMPO the Master Chemical Mechanism (MCMv3.3.1). The GEOS- (Zoogman et al., 2016) will require less temporal averag- Chem mechanism features several updates relevant to CHO- ing and this may reveal the utility of CHOCHO observations CHO formation. These include a decrease in the δ-ISOPO2 + for estimating isoprene emissions under low-NOx conditions NO branching ratio leading to prompt CHOCHO production when isoprene oxidation is titrated. under high-NOx conditions, and a proposed low-NOx pathway for prompt CHOCHO formation by photolysis of a di- hydroperoxide dicarbonyl compound (DHDC) product from Data availability. The SENEX observations used in this paper are (1,5)H-shift isomerization of dihydroperoxy α-formyl per- publicly accessible online (https://esrl.noaa.gov/csd/groups/csd7/ measurements/2013senex/). OMI CHOCHO, HCHO and NO ob- oxy radicals in the ISOPO2 isomerization pathway (Fig.1). 2 GEOS-Chem and MCMv3.3.1 show similar HCHO yields servations can be obtained from the Aura Validation Data Center from isoprene, increasing with increasing NO . CHOCHO (https://avdc.gsfc.nasa.gov/). Details on how to download GEOS- x Chem source code can be found at http://www.geos-chem.org. yields from isoprene in MCMv3.3.1 show behavior similar to HCHO but GEOS-Chem has a higher yield at low NOx from the ISOPO isomerization pathway. 2 Competing interests. The authors declare that they have no conflict Comparison of GEOS-Chem to the SENEX observations of interest. of CHOCHO and HCHO shows good agreement in the boundary layer but a negative CHOCHO model bias in the free troposphere. This could reflect an instrument artifact The Supplement related to this article is available online but may also imply a missing background source in the at https://doi.org/10.5194/acp-17-8725-2017-supplement. model. Mixed layer (< 1 km) observations show a strong CHOCHO–HCHO relationship that is reproduced in GEOS- Chem and is remarkably consistent across all conditions ex-

Atmos. Chem. Phys., 17, 8725–8738, 2017 www.atmos-chem-phys.net/17/8725/2017/ C. Chan Miller et al.: Glyoxal yield from isoprene 8735

Acknowledgements. This work was funded by NASA ACMAP out the troposphere and lower stratosphere, Atmos. Meas. Tech., and ACCDAM and is a contribution to the NASA Aura Science 8, 541–552, https://doi.org/10.5194/amt-8-541-2015, 2015. Team. This research was undertaken with the assistance of Chance, K., Palmer, P. I., Spurr, R. J. D., Martin, R. V., Kurosu, T. P., resources provided at the NCI National Facility systems at the and Jacob, D. J.: Satellite observations of formaldehyde over Australian National University through the National Computational North America from GOME, Geophys. Res. Lett., 27, 3461– Merit Allocation Scheme supported by the Australian Govern- 3464, https://doi.org/10.1029/2000GL011857, 2000. ment. Jennifer Kaiser, Frank N. Keutsch, Glenn M. Wolfe, and Chan Miller, C., Gonzalez Abad, G., Wang, H., Liu, X., Kurosu, Thomas F. Hanisco acknowledge support from the US EPA Science T., Jacob, D. 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